1. Field of the Invention
The present invention relates to an indirect conversion type radiation image detecting device, and a radiation image capturing system having the radiation image detecting device.
2. Description Related to the Prior Art
In a medical field, a radiation image capturing system for imaging the inside of a body cavity becomes widespread. This radiation image capturing system is provided with a radiation source for applying radiation e.g. X-rays to an object, and a radiation image detecting device that converts the radiation passed through the object into electric charge and converts the electric charge into voltage to produce image data representing a radiation image of the object.
There are two types of radiation image detecting devices, that is, a direct conversion type for directly converting the radiation into the electric charge, and an indirect conversion type for temporarily converting the radiation into light (visible light) and converting the light into the electric charge. The indirect conversion type radiation image detecting device has a wavelength conversion layer for converting the radiation into the light, and a solid-state detector for converting the light produced by the wavelength conversion layer into the electric charge. The solid-state detector has a plurality of photodiodes.
The wavelength conversion layer contains a phosphor for converting the radiation into the light. The phosphor is made of particles (hereinafter called phosphor particles) such as GOS (Gd2O2S:Tb), or a columnar crystal such as CsI:Tl. The wavelength conversion layer of particle structure is in widespread use because of being easier to manufacture and less expensive than the wavelength conversion layer of columnar crystal structure. In the wavelength conversion layer of particle structure, the phosphor particles are dispersed into a binder (binding agent) such as resin.
The indirect conversion type radiation image detecting device has the wavelength conversion layer and the solid-state detector stacked in layers. The indirect conversion type radiation image detecting device is categorized in two types depending on which one of the wavelength conversion layer and the solid-state detector to dispose on the side of the radiation source. A method in which the wavelength conversion layer is disposed on the side of the radiation source is referred to as a PSS (penetration side sampling) method. On the contrary, a method in which the solid-state detector is disposed on the side of the radiation source is referred to as an ISS (irradiation side sampling) method (see Japanese Patent Laid-Open Publication No. 2010-112733 and the like).
The light is produced in the wavelength conversion layer upon incidence of the radiation, and the production of the light mainly occurs in a surface on a side upon which the radiation is incident. Thus, according to the PSS method, the light is produced in a surface of the wavelength conversion layer on an opposite side from the solid-state detector, and propagates through the wavelength conversion layer to the solid-state detector. Therefore, since a part of the light is absorbed by the wavelength conversion layer itself or scattered, there is a problem of deterioration in sensitivity (conversion efficiency from the radiation into the light) and sharpness of the image detected by the solid-state detector.
In the ISS method, on the other hand, the radiation passed through the solid-state detector is incident upon the wavelength conversion layer. Since the light is produced in the wavelength conversion layer on the side of the solid-state detector, the ISS method has the advantage that short propagation distance prevents deterioration in the sensitivity and the sharpness, contrarily to above.
For example, in the ISS method, thickening the wavelength conversion layer improves the sensitivity of the wavelength conversion layer. Thickening the wavelength conversion layer, however, causes emission of the light from the phosphor particles at positions far from the solid-state detector. The light emitted from the phosphor particles is widely expanded while propagating to the solid-state detector, so the sharpness of the image deteriorates. Also, for the purpose of improving the sensitivity of the wavelength conversion layer, the size of the phosphor particles is increased and the amount of light emitted from the phosphor particles is increased. In this case, the light that propagates from the phosphor particles to the solid-state detector is expanded further widely and the sharpness further deteriorates.
Thus, the Japanese Patent Laid-Open Publication No. 2010-112733 proposes to constitute the wavelength conversion layer of a stack of a first phosphor layer, which has phosphor particles having a small average particle diameter dispersed in a binder, and a second phosphor layer, which has phosphor particles having a large average particle diameter dispersed in a binder, and dispose the second phosphor layer on the side of the solid-state detector. The second phosphor layer has the large phosphor particles and produces a large amount of light, but the position of each phosphor particle is near the solid-state detector, and hence the expansion of the light is small and the sharpness less deteriorates. In the first phosphor layer, the position of each phosphor particle is far from the solid-state detector, but the size of the phosphor particles are small, and hence the expansion of the light is small and the sharpness less deteriorates. Therefore, this radiation image detecting device can improve the sensitivity without deterioration in the sharpness.
However, according to the radiation image detecting device described in the Japanese Patent Laid-Open Publication No. 2010-112733, in the second phosphor layer disposed on the side of the solid-state detector, the weight of the binder per unit thickness is increased gradually from the side of the first phosphor layer to the side of the solid-state detector, and a spatial filling rate of the phosphor particles is low on the side of the solid-state detector. Thus, this radiation image detecting device is susceptible to improvement in terms of the sharpness.